Release 11 of Long-Term Evolution (LTE)-Advanced provides a control channel known as the Enhanced Physical Downlink Control Channel (EPDCCH). The EPDCCH is transmitted in one or more physical resource block (PRB) pairs in a subframe. The EPDCCH supports both distributed and localized transmission in the frequency domain.
A basic resource unit known as the Enhanced Resource Element Group (EREG) has been defined for the EPDCCH. The mapping of EREGs to resource elements (REs) is fixed in 3rd Generation Partnership Project (3GPP) Long-Term Evolution (LTE) standard specifications. Within each PRB pair, the EREG indices are sequentially mapped to REs, while excluding the REs for demodulation reference signals (24 for normal cyclic prefix (CP) and 12 for extended CP), in a frequency first and then time manner.
Each distributed or localized transmission of control information uses a group of EREGs. Multiple EREGs are sometimes grouped together as Enhanced Control Channel Elements (ECCEs). In this case, each EPDCCH transmission uses one or multiple ECCEs. An ECCE includes N EREGs in both distributed and localized transmissions, where N is equal to 4 or 8. However, the manner in which particular EREGs are grouped together to form ECCEs is undetermined.
An EPDCCH transmission set is defined as a group of P PRB pairs, where P={1 (FFS), 2, 4, 8} for localized transmission, and P={2, 4, 8, 16 (FFS)} for distributed transmission. For PUCCH resource allocation, ECCEs are indexed per EPDCCH transmission set, and the ECCE index is used as one component to determine PUCCH resources. However, ECCE indexing is still an open issue.